MS Teams add in not working on Outlook Classic

This has got me stumped.

So far we've changed registry keys:

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Resiliency\DoNotDisableAddinList
Name: TeamsAddin.FastConnect
Amend name to: TeamsAddin.FastConnect

Ran commands:

%SystemRoot%\System32\regsvr32.exe /s /n /i:user %LocalAppData%\Microsoft\TeamsMeetingAddin\Current\x64\Microsoft.Teams.AddinLoader.dll

Deleted files: 

Press Windows Key + R to open the Run dialog box.Type %localappdata%\Microsoft and hit Enter.Find and delete the folders named TeamsMeetingAddin, TeamsPresenceAddin, and SquirrelTemp (if they exist)

Ran the following:

Close Outlook and Teams.
Open Windows Settings > Apps > Installed Apps (or Programs and Features).
Search for Teams Meeting Add-in or Microsoft Teams Meeting Add-in for Microsoft Office.
Click the three dots (or right-click) and select Uninstall.
Re-open Teams, which will automatically prompt the reinstallation of the add-in, then open Outlook

We've uninstalled and reinstalled Teams. Also Skype is uninstalled.

The RDS Boxes are 2016

And we're stumped as to why MS Teams won't work, and the add-in on the ribbon for Outlook classic also won't enable MS Teams.

Any ideas? Please help!